Selective laser sintering is a relatively new method for producing parts and other freeform solid articles in a layer-by-layer fashion. This method forms such articles by the mechanism of sintering, which refers to any process by which particulates are made to form a solid mass through the application of external energy. According to selective laser sintering, the external energy is focused and controlled by controlling a laser to sinter selected locations of a heat-fusible powder. By performing this process in layer-by-layer fashion, complex parts and freeform solid articles which cannot be fabricated easily (if at all) by subtractive methods such as machining can be quickly and accurately fabricated. Accordingly, this method is particularly beneficial in the production of prototype parts, and is particularly useful in the customized manufacture of such parts and articles directly from computer-aided-design data bases.
Selective laser sintering is performed by depositing a layer of a heat-fusible powder onto a target surface; examples of the types of powders include metal powders, polymer powders such as wax that can be subsequently used in investment casting, ceramic powders, and plastics such as ABS plastic, polyvinyl chloride (PVC), polycarbonate, and other polymers. Portions of the layer of powder corresponding to a cross-sectional layer of the part to be produced are exposed to a focused and directionally controlled energy beam, such as generated by a laser having its direction controlled by mirrors, under the control of a computer. The portions of the powder exposed to the laser energy are sintered into a solid mass in the manner described hereinabove. After the selected portions of the layer have been so sintered or bonded, another layer of powder is placed over the layer previously selectively sintered, and the energy beam is directed to sinter portions of the new layer according to the next cross-sectional layer of the part to be produced. The sintering of each layer not only forms a solid mass within the layer, but also sinters each layer to previously sintered powder underlying the newly sintered portion. In this manner, the selective laser sintering method builds a part in layer-wise fashion, with flexibility, accuracy, and speed of fabrication superior to conventional machining methods.
The selective laser sintering process, and apparatus for performing the process, is described in further detail in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, U.S. Pat. No. 4,938,816, issued Jul. 3, 1990, U.S. Pat. No. 4,944,817, issued Jul. 31, 1990, and PCT Publication WO 88/02677, published Apr. 21, 1988, all of which are incorporated herein by this reference.
A problem faced by those in the selective laser sintering field is the warpage and shrinkage of the part due to thermal effects. Such warpage may manifest as the curling of a sintered layer in such a manner that it does not bond to the previously sintered layer directly therebelow; another manifestation of this warpage occurs even though the layers of the part bond together, but where the part itself warps, for example where a bottom flat surface curls up at the edges to become a curved surface, concave up. It is believed that a significant cause of this warpage is the thermal shrinkage of the sintered layer from its temperature during sintering to its post-sintering temperature, an extreme case of which causes the individual layers to not bond to one another. In addition, uneven cooling of the part during its layer-wise manufacture, for example where top layers of the part are cooled more quickly than bottom layers, has been observed to cause warpage and curling.
It has been observed that control of the temperature of the article being produced is an important factor in reducing such warpage. An apparatus for controlling the part temperature is described in the above-referenced PCT Publication WO 88/02677, which provides a draft of temperature-controlled air through the target area (i.e., through the powder and the part being produced). Such control by this draft is believed to reduce the temperature differential that the part is exposed to during and after the sintering process, reducing shrinkage from cooling, and to maintain the temperature of previously sintered layers at high enough temperature to allow relaxation.
Another significant problem faced by those in the field of selective laser sintering is undesired growth of the part being produced beyond the volume defined by the energy beam. As is well known, the spot size of a laser beam can be made quite small, so that according to the selective laser sintering method which defines the volume of the part by the laser scan, the resolution of the part being produced can theoretically be quite high. However, conduction of heat resulting from the sintering can cause particles of the powder outside the laser scan to sinter to the directly sintered portion. This causes the cross-sectional layer to be larger than that defined by the laser scan. In addition, growth can occur from layer to layer, for example where sufficient heat from sintering remains in the sintered portion of the layer at the time that the next layer of powder is disposed thereover, so that the next powder layer sinters to the prior layer without exposure to the laser beam. The downdraft apparatus described hereinabove was found to provide transfer of bulk heat from the layer being sintered, reducing the extent of such interlayer growth.
However, the use of convection temperature control is limited in its accuracy in uniformly controlling the temperature of the layer being produced. This is due to the undefined and non-uniform path that the air draft necessarily must follow as it passes through the part being produced (the definition of the part controlling the path of the draft). Accordingly, another technique that has been used in attempts to control the temperature of the part being produced has been radiant heaters placed near the target surface. Such radiant heaters have included floodlamps, quartz rods, and conventional flat radiant panels.
Referring to FIG. 1, a prior apparatus including flat radiant panels in providing radiant heat to the selective laser sintering target surface will now be described. The apparatus shown in FIG. 1 is a schematic representation of the SLS Model 125 DeskTop Manufacturing system manufactured and sold by DTM Corporation. The apparatus of FIG. 1 includes a chamber 2 (front doors and the top of chamber 2 are not shown in FIG. 1, for purposes of clarity), within which the selective sintering process takes place. Target surface 4, for purposes of the description herein, refers to the top surface of heat-fusible powder (including portions previously sintered, if present) disposed on part piston 6. The vertical motion of part piston 6 is controlled by motor 8. Laser 10 provides a beam which is reflected by galvanometer-controlled mirrors 12 (only one of which is shown for clarity), in the manner described in the U.S. Patents referred to hereinabove. Powder piston 14 is also provided in this apparatus, controlled by motor 16. As described in the above-referenced PCT Publication 88/02677, counter-rotating roller 18 is provided to transfer the powder to the target surface 4 in a uniform and level fashion.
In operation, the apparatus of FIG. 1 supplies powder to chamber 2 via powder cylinder 14; powder is placed into chamber 2 by the upward partial motion of powder cylinder 14 provided by motor 16. Roller 18 (preferably provided with a scraper to prevent buildup, said scraper not shown in FIG. 1 for clarity) spreads the powder within the chamber by translation from powder cylinder 14 toward and across target surface 4 at the surface of the powder on top of part piston 6, in the manner described in said PCT Publication 88/02677. At the time that roller 18 is providing powder from powder piston 14, target surface 4 (whether a prior layer is disposed thereat or not) is preferably below the floor of chamber 2 by a small amount, for example 5 mils, to define the thickness of the powder layer to be processed. It is preferable, for smooth and thorough distribution of the powder, that the amount of powder provided by powder cylinder 14 be greater than that which can be accepted by part cylinder 6, so that some excess powder will result from the motion of roller 18 across target surface 4; this may be accomplished by the upward motion of powder piston 14 by a greater amount than the distance below the floor of chamber 2 that target surface 4 is set at (e.g., 10 mils versus 5 mils). It is also preferable to slave the counter-rotation of roller 18 to the translation of roller 18 within chamber 2, so that the ratio of rotational speed to translation speed is constant.
Further in operation, after the transfer of powder to target surface 4, and the return of roller 18 to its original position near powder piston 14, laser 10 selectively sinters portions of the powder at target surface 4 corresponding to the cross-section of the layer of the part to be produced, in the manner described in the above-referenced U.S. Patents and PCT Publication. After completion of the selective sintering for the particular layer of powder, part piston 6 moves downward by an amount corresponding to the thickness of the next layer, awaiting the deposition of the next layer of powder thereupon from roller 18.
Radiant heat panels 20 are provided in this prior apparatus of FIG. 1, suspended from the roof of chamber 2 (in a manner not shown). Radiant heat panels 20 in this prior arrangement are conventional flat rectangular heat panels, each of which emit energy per unit area substantially uniformly across its surface. In this arrangement, radiant heat panels 20 are separated from one another to allow the beam from laser 10 to pass therebetween, and are disposed at an angle relative to target surface 4, to heat target surface 4 so that the surface temperature can be controlled to reduce growth and curling, as described hereinabove.
Temperature non-uniformity at target surface 4 has been observed in use of the arrangement of FIG. 1. Such non-uniformity in target surface temperature can allow growth at one portion of the part being produced (i.e., at the hottest location) simultaneously with curling or other warpage at another portion of the part (i.e,. at the coolest location). Accordingly, for the apparatus of FIG. 1, this non-uniformity makes it difficult to optimize the temperature at the surface target 4 in order to keep either of these deleterious effects from occurring.
It should also be noted that uniform radiant heating of a surface may theoretically be accomplished by providing a flat radiant heating element placed parallel to the surface being heated, and of effectively infinite size relative to the target surface. In the use of such a heater in a closed chamber, however, it is not practicable to provide such a large heater, as excessively large chambers reduce the ability to control the ambient temperature therein, and are not preferred for commercial applications due to the associated cost of requiring a large "footprint". It should also be noted that such a flat heater is necessarily not compatible with selective laser sintering, as no opening for the laser is provided therethrough.
It is therefore an object of this invention to provide a radiant heater which delivers energy to a substantially planar surface distanced therefrom in such a manner that the total energy per unit area incident upon the planar surface is substantially uniform.
It is a further object of this invention to provide such a heater which results in uniform temperature at a planar surface distanced therefrom.
It is a further object of this invention to provide such a radiant heater having an opening in its center.
It is a further object of this invention to provide such a radiant heater which is particularly adapted for use in an apparatus for selective laser sintering.
It is a further object of this invention to provide an apparatus for selective laser sintering having such a radiant heater.
It is a further object of this invention to provide such a radiant heater having controllable segments therein, to allow for adjustment of the surface temperature.
Other objects of this invention will be apparent to those of ordinary skill in the art having reference to the following specification, together with the drawings.